The invention relates to pressure-operated rotary clutches for use in a geared power transmission mechanism.
Automatic transmissions for use in contemporary vehicles typically include multiple-ratio gearing that defines plural torque flow paths between the vehicle engine and the vehicle traction wheels. The torque input shaft for the gearing is connected to the engine. A hydrokinetic torque converter may establish the connection. The impeller of the torque converter is connected to the engine crankshaft, and the turbine of the torque converter is connected to a torque input element of the gearing. Relative motion of the elements of the gearing is controlled by fluid pressure-operated rotary clutches and brakes.
The rotary clutches, which establish and disestablish torque flow paths through the gearing, include an annular cylinder connected to the torque input element of the gearing and an annular piston received in the cylinder. The cylinder and the piston define an annular pressure chamber which, when pressurized with a command pressure, establishes a piston force that frictionally engages clutch discs to effect torque distribution from the torque input element to the gearing.
The rotation of the clutch and piston assembly creates a centrifugal pressure in the annular pressure chamber. When a clutch control pressure is applied to the annular pressure chamber to effect a desired clutch torque-carrying capacity, the centrifugal pressure developed in the pressure chamber must be offset so that an accurate correlation can be established between the commanded pressure and the clutch torque-carrying capacity.
It is known design practice to provide an annular fluid dam in the rotating clutch and piston assembly and to supply the annular fluid dam with lubricating oil, which rotates in unison with the clutch cylinder. The lubricating oil, as in the case of the fluid in the clutch pressure chamber, is subjected to centrifugal pressure. That centrifugal pressure tends to establish a centrifugal pressure force that opposes the centrifugal pressure force established in the clutch pressure chamber. The net clutch apply force then is approximately equal to the force created by the commanded pressure force.
Another design technique for compensating for the effect of centrifugal pressure buildup in a rotating clutch assembly involves the use of a ball check valve assembly in the annular cylinder, which is closed when the annular cylinder is pressurized but which opens under the effect of centrifugal force due to the mass of a ball check valve element when the cylinder is depressurized as residual fluid in the cylinder is exhausted.
Each of these prior art designs presents undesirable performance consequences. The use of an annular lubricating fluid dam, for example, requires an increase in the axial dimension of the rotating clutch assembly as well as fluid feed passage structure that is dedicated to the supply of lubricating oil to the fluid dam. In the case of a ball check valve assembly that relieves pressure from the clutch assembly, a clutch apply delay following a command for clutch application may present an unacceptable control problem. Further, the fluid displaced through the ball check valve assembly when the clutch is released must be replaced when the clutch is reapplied, thereby introducing a design variable that must be accounted for in the clutch application and release control strategy.
It is an objective of the invention to provide a rotary clutch assembly with a centrifugally-balanced piston without the need for using a ball check valve assembly or a rotary fluid dam, as in the case of prior art constructions. It includes a rotary clutch cylinder that cooperates with a clutch piston to define a pressure release chamber and a pressure apply chamber that provide essentially complete centrifugal balance.
The entire clutch assembly, as well as the friction elements of the clutch, are subjected to rotation. The clutch piston and the cooperating cylinder define dual piston areas, which permits development of a large gain with a single clutch in a given gear ratio, thus increasing the packaging efficiency of the gearing and the clutch assembly in a transmission environment. The fluid in the rotating cylinder assembly is subjected to centrifugal loads, creating a pressure head. The force created by this pressure head is combined with the other forces acting on the piston.
The dimensions of the pressure-apply chamber and the pressure-release chamber are chosen such that the centrifugal forces are essentially balanced. A dimensional balance coefficient is established for each of the chambers. The effective areas on the piston for the release chamber and for the apply chamber, as well as the dimensions of the release chamber and the apply chamber, are chosen in the rotating clutch assembly of the invention so that the dimensional balance coefficient for one cylinder is equal to the dimensional balance coefficient for the other cylinder. If a relatively large gain is desired, only the apply cylinder is pressurized. If a reduced gain is desired, both chambers are pressurized with a common feed pressure.
A return spring is located in the rotating clutch assembly. The spring force created by the return spring urges the piston to a clutch-release position. The clutch stroke pressure thus can be maintained at a relatively low level because the piston is centrifugally balanced and the spring can be compressed without the necessity for overcoming a centrifugal pressure head in the clutch assembly. This makes it possible to use a low rate spring rather than a very stiff spring.
Since the piston is hydraulically balanced, there is no tendency for a centrifugal pressure to create a clutch actuating force when the piston is rotated at high speeds when clutch application is not commanded. When the clutch pressure is commanded by the transmission control system to establish a torque flow path through the gearing, a reliable commanded force can be established as the clutch apply pressure is compensated for the effect of the net centrifugal force acting in the clutch-release chamber and the clutch-apply chamber. This makes it possible to achieve a constant, speed-independent, net clutch-apply force throughout the entire cylinder speed range. This is accomplished without the necessity for accounting for uncontrollable variables such as seal friction and spring force and for other non-linearities that would be present in a rotary clutch assembly of conventional design.
An embodiment of the invention has a first rotary clutch member that defines an apply pressure cylinder. A clutch-apply piston is in the apply pressure cylinder. An apply pressure chamber is defined by the clutch-apply piston and the apply pressure chamber. The clutch-apply piston and a second rotary clutch member in the clutch-apply piston define a release pressure chamber. The clutch return spring is between the second rotary clutch member and the clutch-apply piston. Each pressure chamber is characterized by a centrifugal pressure balance coefficient, the coefficients being approximately equal.